As well known to those skilled in the art, it is possible to remove water from mixtures thereof with organic liquids by various techniques including adsorption or distillation. These conventional processes, particularly distillation, are however, characterized by high capital cost. In the case of distillation for example the process requires expensive distillation towers, heaters, heat exchangers (reboilers, condensers, etc.), together with a substantial amount of auxiliary equipment typified by pumps, collection vessels, vacuum generating equipment, etc.
Such operations are characterized by high operating costs: principally costs of heating and cooling - plus pumping, etc.
Furthermore the properties of the materials being separated, as is evidenced by the distillation curves, may be such that a large number of plates may be required, etc. when the material forms an azeotrope with water, additional problems may be present which, for example, would require that separation be effected in a series of steps (e.g., as in two towers) or by addition of extraneous materials to the system.
There are also comparable problems which are unique to adsorption systems.
It has been found to be possible to utilize membrane systems to separate mixtures of miscible liquids by pervaporation. In this process, the charge liquid is brought into contact with a membrane film; and one component of the charge liquid preferentially permeates the membrane. The permeate is then removed as a vapor from the downstream side of the film--typically by sweeping with a carrier gas or by reducing the pressure below the saturated vapor pressure of the permeating species.
Illustrative membranes which have been employed in prior art techniques include those set forth in the following table:
TABLE ______________________________________ Separating Layer References ______________________________________ Nafion brand of Cabasso and Liu perfluorosulfonic acid J. Memb. Sci. 24, 101 (1985) Sulfonated polyethylene Cabasso, Korngold & Liu J. Pol. Sci Letters, 23, (1985) Fluorinated polyether USP 4,526,948 to Dupont as or Carboxylic Acid fluorides assignee of Resnickto Selemion AMV brand of Wentzlaff Boddeker & Asahi Glass cross-linked Hattanback, J. Memb. Sci. styrene butadiene (with 22, 333 (1985) quaternary ammonium residues on a polyvinyl chloride backing) Cellulose triacetate Wentzlaff, Boddeker & Hattanback, J. Memb. Sci. 22, 333 (1985) Polyacrylonitrile Neel, Aptel & Clement Desalination 53, 297 (1985) Crosslinked Polyvinyl Alcohol Eur. Patent 0 096 339 to GFT as assignee of Bruschke Poly(maleimideacrylonitrile) Yoshikawa et al J. Pol. Sci. 22, 2159 (1984) Dextrine - isophorone- Chem. Econ. Eng. diisocyanate Rev., 17, 34 (1985) ______________________________________
The cost effectiveness of a membrane is determined by the selectivity and productivity. Of the membranes commercially available, an illustrative membrane of high performance is that disclosed in European Patent 0 096 339 A2 of GFT as assignee of Bruschke--published 21 December 1983.
European Patent 0 096 339 A2 to GFT as assignee of Bruschke discloses, as cross-linking agents for the polyvinyl alcohol membrane, diacids (typified by maleic acid or fumaric acid); dihalogen compounds (typified by dichloroacetone or 1,3-dichloroisopropanol); aldehydes, including dialdehydes, typified by formaldehyde. These membranes are said to be particularly effective for dehydration of aqueous solutions of ethanol or isopropanol.
This reference discloses separation of water from alcohols, ethers, ketones, aldehydes, or acids by use of composite membranes. Specifically, the composite includes (i) a backing typically about 120 microns in thickness, on which is positioned (ii) a microporous support layer of a polysulfone or a polyacrylonitrile of about 50 microns thickness, on which is positioned (iii) a separating layer of cross-linked polyvinyl alcohol about 2 microns in thickness.
U.S. Pat. No. 4,728,429 to Cabasso et al, U.S. Pat. No. 4,067,805 to Chiang et al, U.S. Pat. No. 4,526,948 to Resnick, U.S. Pat. No. 3,750,735 to Chiang et al, U.S. Pat. No. 4,690,766 to Linder et al, and U.S. Pat. No. 4,798,674 to Pasternak, Bartels and Reale, Jr. provide additional background. See also Yamamoto et al Japanese 61/161109 A2 of 21 July 1986 Preparation of Separation Membranes.
It is an object of this invention to provide a novel composite membrane characterized by its ability to effect separation of water from organic oxygenates such as isopropyl alcohol. Other objects will be apparent to those skilled in the art.